We are pleased to announce support for cross-zone load balancing, which changes the way that Elastic Load Balancing (ELB) routes incoming requests, making it even easier for you to deploy applications across multiple Availability Zones.

Until now, ELB relied on DNS to distribute requests from clients to the Availability Zones where the back-end instances were located. This meant that you could experience situations where some instances would receive a disproportionally higher amount of inbound requests than others because clients were incorrectly caching DNS information. In addition, if you did not have an equal number of healthy instances in each of the Availability Zones (e.g. if you had taken some instances down for maintenance), your requests would be balanced across a smaller number of healthy back-end instances in the affected Availability Zones – meaning that you would have to closely monitor and adjust your back-end instance capacity.

If you enable cross-zone load balancing, you no longer have to worry that clients caching DNS information will result in requests being distributed unevenly. And now, ELB ensures that requests are distributed equally to your back-end instances regardless of the Availability Zone in which they are located. This change makes it easier for you to deploy and manage your applications across multiple Availability Zones.